The roles of morphogenetic differentiation in controlling the phenotypic expression of neoplastic transformation, the degree of malignancy of tumors, and the susceptibility of developing organs to carcinogenesis are studied using organ culture and tissue transplantation techniques, with current emphasis on the kidney. The presence of genotoxic damage in target and non-target tissues exposed transplacentally to chemical carcinogens during embryonal and fetal development is assayed by detection of mutations at 3 loci in cells cultured from embryos exposed in utero and is correlated with organ specificity for chemical carcinogenesis by the same agents.